The present invention relates to a wastewater treatment system, and more particularly to a wastewater collection and discharge system for limiting the discharge of solids from a septic tank.
A typical wastewater treatment system which is operated by a homeowner or a small sanitary district includes a wastewater collection container such as a septic tank for primary treatment, followed by a sand filter, a mound, a gravity flow or pressurized drain field, drip irrigation, or a sand-filled drain field bed or trench for secondary treatment of the effluent from the container. Other systems which use a septic tank for primary treatment include septic tank-effluent pump pressure sewer systems and small diameter gravity sewers. The present invention is applicable to all such systems.
In a conventionally operated septic tank, raw sewage wastewater having a significant concentration of waste solids enters one end of the tank. In the septic tank, solids separate from the liquid portion of the sewage; solids having a lower density than the liquid move to the top of the liquid to form a scum layer, and the solids having a higher density than the liquid sink to the bottom of the tank to form a sludge layer, resulting in a relatively clear horizontal liquid layer between the scum and the sludge layers which gradually narrows as the scum and sludge layers accumulate and thus converge toward each other. The liquid portion of the wastewater is drawn from this liquid layer and discharged from the tank as the septic tank effluent by means of gravity, a pump, or a siphon.
The secondary treatment of the discharged effluent is frequently determined by selecting from the various alternatives the most economical method of obtaining an acceptably disposable effluent. A drain field is frequently the preferred type of secondary treatment because of the relative costs of the various alternatives, but the land available for the drain field may be geographically unsuitable for receiving septic tank effluent of the quality currently produced by available septic tanks. The quality of septic tank effluent is significantly improved by preventing the discharge of solids with the liquid effluent. The larger unintentionally discharged solids are troublesome when present in septic tank effluent because they impart a solids loading to the secondary treatment or disposal facilities. In addition, many septic tank effluent pumps are not capable of passing gross solids. Furthermore, solids can cause clogging in the small diameter orifices which are used in the piping systems of many pressurized systems.
It is known to surround the outlet of a septic tank with a cylindrical screen to prevent gross solids from being discharged with the effluent. Some septic tank effluent screens are made from a mesh having a square mesh opening of about 1/8 inch. However, if such a mesh screen is used where a high proportion of solids is encountered, the mesh becomes clogged. Also, some stringy solids become woven into the mesh. When such a screen becomes clogged the homeowner or maintenance provider must remove and clean the screen because none of the known screens can be cleaned while remaining in place in the septic tank. In the most severe of cases, a clogged screen may result in a service interruption until such maintenance can be performed.
As solids digest in a septic tank considerable gas is produced, about one cubic foot per capita-day. The gas bubbles become attached to solid particles and lift them from the sludge layer toward the scum layer. Similarly, a heavier-than-water solid that was gas lifted to the scum layer settles back down toward the sludge layer when the gas bubble is released. Thus, these mobile solids pass through the clear layer between the sludge and scum and can escape with the effluent drawn from that layer. When the screen surrounding the outlet of the septic tank has a closed bottom a screening action occurs as the effluent passes into the screen prior to exiting the septic tank outlet conduit. However, as noted, these screens tend to clog. When such a screen has an open bottom the screening action occurs as the effluent exits the screen. However, mobile solids rise vertically through the bottom of the screen and cause the interior of the screen to become clogged. In an attempt to overcome the clogging problem common to the mesh screens, screens with horizontal slot-shaped openings have been used. However, the solids present in the septic tank are often flat, not spherical, and as the solids travel most of them tend to be horizontally or vertically oriented. The horizontally-oriented mobile solids are not effectively screened out of the effluent by the horizontally slotted screens, which severely limits the effectiveness of the horizontally slotted screens.
A standard septic tank outlet withdraws effluent from a predetermined level coinciding with the level where the effluent is expected to be most free of solids. This is typically the expected level of eventual convergence of the top of the sludge layer and the bottom of the scum layer. The calculation of that predetermined level is based on sludge and scum layer accumulations of an average household. However, it has been shown that sludge and scum layers accumulate at widely varying rates. Consequently, it is rare that the level from which effluent is drawn is properly established for the home actually being served. Unless the tank is pumped often, sludge or scum encroaches prematurely on the outlet and is carried out with the effluent, imposing an undesirable solids loading on the secondary treatment or disposal facility as described previously, and possibly clogging such facility and its fluid conduits. Furthermore, in a conventionally operated septic tank the homeowner cannot predict when pumping of the tank may be necessary, and the family is at risk for an inconvenient service interruption and damage to the secondary treatment or disposal facility.
Thus, a need exists for an improved system for limiting the solids discharged from a wastewater collection container such as a septic tank while compatibly overcoming the problems of prior systems.
According to one aspect of the present invention, such need is satisfied by immersing a substantially upstanding elongate tube in the wastewater within a septic tank or comparable container, with the interior of the tube exposed to the clear horizontal layer of liquid through a plurality of apertures in the tube which prevent the passage of larger solids. The liquid flows from the horizontal layer through the apertures into the interior of the tube and thereafter through a liquid effluent outlet of the container. A cleaning implement is moved along the interior of the tube while the tube remains immersed in the wastewater within the container thereby cleaning the tube and its apertures. Thus, the tube can be cleaned without removing it from the waste-water container. If the tube is permanently installed in the container, the homeowner is never tempted to remove the tube and discard it rather than perform the necessary routine maintenance.
According to another aspect of the invention, the exterior surface of a removable tube contacts not only the horizontal layer of liquid, but also at least the sludge layer and possibly also the scum layer depending upon use conditions. The tube is removed from the container and the exterior surface of the tube is inspected to identify the portions thereof which have been in contact with the solids layer or layers. Thus, the maintenance provider can remove and inspect the tube and determine the extent to which a solids layer is encroaching on the liquid layer. Regular inspections provide a measure of the rate of growth of the solids layers, so that the homeowner can easily determine if the wastewater container requires pumping.
According to another aspect of the invention, the apertures in the tube extend vertically along a majority of the depth of the wastewater in the container and contact at least the liquid layer and sludge layer while the tube substantially prevents the exposure of the interior of the tube to the scum layer through an upper extremity of the tube despite an overflow condition in the container. Thus, the apertures are not limited to a predetermined liquid level where premature encroachment of solids on the apertures may occur from unexpected rates of sludge and scum accumulation leading to undesirable solids loading on the secondary treatment or disposal facility. Instead, at least some apertures will normally always be open to the clear liquid layer, from which the effluent will continue to be withdrawn since such apertures will offer the least resistance to effluent flow. Even in the event of an unusually high liquid level in the container, access of solids to the interior of the tube will nevertheless be limited to access through the apertures.
According to another aspect of the invention, liquid flows from the horizontal liquid layer into the interior of the tube through apertures which comprise a plurality of elongate slots in the tube oriented longitudinally at an inclination. The inclined slots effectively exclude the majority of the mobile solids present in the horizontal layer of liquid because those mobile solids are oriented vertically or horizontally.
According to another aspect of the invention, liquid flows from the interior of the tube to the outlet through a first opening in the tube having a first width, thereby providing a restriction on the volumetric flow rate of the liquid. The liquid can also flow from the interior of the tube to the outlet through a second opening in the tube located at a higher elevation than the first opening and having a second width greater than the first width, thereby lessening the restriction. Thus, the first opening provides desirable flow attenuation of the container effluent but, if the first opening becomes clogged, the second opening nevertheless permits effluent to exit the container.
According to a further aspect of the invention, liquid flows from the horizontal liquid layer through the apertures into the interior of the tube and thereafter not only through the normal liquid effluent outlet but also through an overflow outlet in parallel with the normal outlet to an alternative disposal site if the normal outlet should temporarily be unable to accept effluent.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.